The boys and their mother were visiting a friend who lived in their complex.

At some point, the boys moved into a back bedroom, Payson Police Chief Donald Engler said.

About 15 minutes into the visit, a shot rang out.

"Officers arrived and found a mother carrying a child into the parking lot, where she met medical personnel," Engler said.

"We have confirmed at this point that it was the 3-year-old that was in control of the weapon at the time that it discharged, and it was the 1½-year-old child that was struck by the round from the weapon," he said.

The boy was taken to Payson Regional Medical Center, where he died.

Engler said the investigation was ongoing.

Last month, a 2-year-old Utah boy died after his 3-year-old sister shot him in the stomach with a rifle their father left in the living room, the Associated Press reported.

Investigators there believed the shooting was accidental but planned to submit a report to county prosecutors for review.